In recent years, advanced signal processing is required for a variety of communication systems, leading to a need for integrated circuits which are capable of wideband signals. In particular, the transmission rate is remarkably increased in optical communication systems, and a 2.4 Gigabits/second (Gb/s) system and a 10 Gb/s system have been brought into practical use. Further, research and developments have been in progress for systems having a transmission rate of 40 Gb/s or higher.
Multiplexed signals transmitted in such an optical communication system include frequency components which span a wide band from several tens of kilohertz (kHz) to several tens of Gigahertz (GHz). For this reason, amplifiers used in transceivers are required to provide a flat gain over a wide band from several tens of kHz to several tens of GHz. A distributed amplifier (for example, see JP-6-125224-A) is an example of such a wideband amplifier.
FIG. 1 is a circuit diagram showing an exemplary configuration of a distributed amplifier. Referring to FIG. 1, the distributed amplifier comprises, by way of example, a plurality of field effect transistors (FET) 45, input side termination resistor 46, input side coupling circuit 47, output side termination resistor 48, and output side coupling circuit 49. Input side coupling circuit 47 comprises a plurality of distributed constant lines 43 connected in series. Output side coupling circuit 49 comprises a plurality of distributed constant lines 44 connected in series.
One terminal of input side coupling circuit 47 is input terminal 41, and the other terminal is connected to one terminal of input side termination circuit 46. Also, the other terminal of input side termination resistor 46 is grounded. Then, input terminal 52 of each of the plurality of FETs 45 is connected to each connection of distributed constant lines 43, which are commented to each other, in input side coupling circuit 47.
Output terminal 53 of each of the plurality of FETs 45 is connected to each connection of distributed constant lines 44, which are connected to each other, in output side coupling circuit 49. Then, A terminal further away from input terminal 41 of output side coupling circuit 49 is output terminal 42, and the other terminal is connected to one terminal of output side termination resistor 48. The other terminal of output side termination resistor 48 is grounded.
In such a distributed amplifier, distributed constant line 43 and capacitance Cgs between gate and source of FET 45 adjacent thereto form a pseudo-distributed constant line which has characteristic impedance Zg. Also, source-drain capacitance Cds of each FET 45 and distributed constant line 44 adjacent thereto form a pseudo-distributed constant line.
Next, a description will be given of the operation of the distributed amplifier shown in FIG. 1.
A signal applied from input terminal 41 propagates through a plurality of distributed constant lines 43 toward input side termination resistor 46. Most of the propagating signal is distributed to each FET 45 in sequence, and amplified therein.
On the other hand, useless signal which has not been distributed to any of FETs 45 are absorbed by input side termination resistor 46. In this way, input side coupling circuit 47 in such a configuration generally exhibits a good input reflection characteristic over a wide band, without using a matching circuit.
On the other hand, the signal applied to each FET 45 is amplified in accordance with the gate width of each FET 45. The signal amplified by each FET 45 propagates toward output terminal 42 through a plurality of distributed constant lines 44 in output side coupling circuit 49, and is sequentially combined and delivered from output terminal 42. Each of propagation paths from input terminal 41 to output terminal 42 is configured to have an electric length equal to each other. Such output side coupling circuit 49 exhibits a good reflection characteristic over a wide band without using a matching circuit in a configuration similar to input side coupling circuit 47.
In the distributed amplifier as described above, a bias tee is generally used as a circuit for applying a bias to each FET 45. FIG. 2 is a circuit diagram showing an exemplary circuit configuration which employs a bias tee in the distributed amplifier of FIG. 1. Referring to FIG. 2, bias tee 56 comprising inductor 54 and capacitor 55 is used for each of input terminal 41 and output terminal 42.